In the metal casting industry, it is customary to employ metal casting vessels, such as tundishes and ladles, etc., as means which serve to transfer various molten metals. Because of the corrosive nature of the liquid metals and the slags, and to prevent heat loss and premature solidification of the metals, the metal casting vessels are prevented from contacting with such metals and/or slags by lining the vessels with heat-insulating refractory boards. Additionally, a trend in the industry is to preheat these lined vessels to minimize heat loss from the initial molten metals poured through the vessels at the start of casting, and to remove, if possible, all sources of hydrogen derived from, for instance, moisture (H.sub.2 O) and/or organic compounds embodied in the refractory linings, which can be dissolved by and incorporated into the liquid metals passing through the lined vessels. In particular, when low hydrogen grades of steel are being cast, it is especially desirable to preheat such refractory lined vessels to remove all possible hydrogen sources which can serve only to contaminate the liquid metals.
In addition to driving off all sources of hydrogen, it is desirable to minimize the amount of unstable oxides, such as silica, which are present in the heat-insulating refractory boards. These unstable oxides, and in particular silica, can react with various elements contained in the molten metals and lead to the formation of oxide inclusions in the liquid metals. For example, some of the undesirable reactions of silica with various elements leading to the formation of oxide impurities are as follows: EQU 3SiO.sub.2 +[4AL].fwdarw.2AL.sub.2 O.sub.3 +[Si] EQU SiO.sub.2 +[2Fe].fwdarw.2FeO+[Si]
The MnO and FeO formed can further attack the silica in the heat-insulating refractory linings by forming low melting liquid oxide slags at metal casting temperatures.
Unfortunately, the dilemma facing the metal-making industry concerning the addition of unstable oxides which act to lower sintering and solidus temperatures versus the use of pure, stable refractory oxides for refractoriness and molten metal purity is extremely difficult to overcome, especially with preheatable heat-insulating refractory boards which must sinter and develop sufficient hot strength for casting at sub-casting temperatures which can be sometimes as much as about 1,000.degree. F. lower than casting temperatures.
Another problem presently associated with the heat-insulating refractory linings for the metal casting vessels involves shrinkage of such linings upon heating. One solution within the metal-making industry to this problem is to fire such linings at temperatures higher than those that are expected during use, so that shrinkage during use can be avoided. Again, since preheating can occur at temperatures as low as about 1000.degree. F. below casting, this presents a further problem with the current preheatable boards.
In the case of cold tundish practice, i.e., the pouring of a molten metal into a tundish without first preheating it, the temperature increases as the molten metal enters the tundish and decomposes the organic binder under reducing conditions forming carbon bonds. The carbon bonds hold the refractory grain together giving the tundish lining the required hot strength. As the carbon bonds are dissolved by the molten metal and oxidized, sintering of the refractory grain occurs over time. Thus, in cold tundish practice, the organic binder decomposition gives carbon bonds allowing the use of more stable refractory oxides which sinter more slowly and at higher temperatures, i.e., MgO and silica.
Nevertheless, in casting, the temperature associated with cold tundish linings increases quickly to that of casting such that by the time the carbon bonds are completely disintegrated, the linings are still held together by the formation of ceramic bonds resulting from the sintering of the refractory oxides. Because preheating may sometimes last up to, for example, 12 hours before casting actually begins, the linings utilized in cold tundish practice are unsuited for preheating use. The problem basically is due to oxidation of the carbon bonds within the linings at preheat temperatures which are generally too low for ceramic bonds to form resulting in usually soft and weak linings which will collapse due to their own weight or wash away as the molten metal enters the vessels.
In the past, several attempts or approaches without success have been made to overcome the problems presently associated with preheatable heat-insulating refractory boards for metal casting vessels. For example, large amounts of low-melting glass formers, such as borax, have been incorporated into the linings in an effort to stick the refractory grain together at preheat temperatures. Unfortunately, the glassy or liquid bonds allow the preheated linings to be deformed easily at preheat and casting temperatures after the carbon bonds burn out. Further, the preheated linings generally fail to develop the requisite hot strength for casting when preheated at preheat temperatures for extended periods of time prior to the start of casting. As a result, it has generally been found that at both preheat and casting temperatures, the liners would collapse or wash away.
An additional problem associated with the use of low-melting glass formers is that they are generally thermodynamically unstable to, for instance, ferrous alloys. In the case of B.sub.2 O.sub.3, it can be reduced resulting in the incorporation of boron into the molten metals, such as ferrous alloys, that can alter the properties of the ferrous alloys as well as produce oxide inclusions.
Other types of preheatable linings are those made with the addition of about 5% to about 20% quartz (silica) for the purpose of bonding with MgO. Unfortunately, these preheatable boards have two serious drawbacks. First, the addition of quartz or other silica forms utilized by these liners is sufficiently high enough to cause formation of oxide inclusions by reaction of the molten metals with the linings. In order to minimize liquid metal contamination, the metal manufacturers specify that the quartz or free silica levels should be as low as possible. Secondly, presence of finely divided quartz or free crystalline silica can become airborne when, for instance, the boards are removed from the vessels after use presenting health hazards to the metal manufacturers and workers.
Examples of still other types of preheatable linings are those which contain about 85%-90% magnesite and about 5% to about 10% calcium fluoride. The calcium fluoride is typical of a strong fluxing agent which reacts with oxides to develop a liquid bonding phase at preheat temperatures. These linings, like those utilizing the low-melting glass formers, develop a liquid bonding phase when the organic binder is burnt out at, for instance, 1900.degree. F. and up (preheat temperatures). The linings, unfortunately, are also very soft and weak at such temperatures after the organic binder is oxidized. Thus, as with the preheatable linings containing low melting glass formers, these preheatable linings fail to develop the sufficient hot strength for casting when heated at preheat temperatures for typical preheat periods of time.
In summary, previous attempts or approaches have been made to develop suitable preheatable insulating refractory liners. Heretofore no satisfactory preheatable heat-insulating refractory liner has been developed which can overcome the problems aforementioned. Basically, the past preheatable liners fall into two categories: those in which quartz is added in unacceptable amounts to form a ceramic bond; and those in which low melting materials are added to develop a liquid phase at preheat temperatures as an unsatisfactory attempt to protect the carbon bonds from oxidation and to bond the refractory grain and promote sintering.
In other words, all of the preheatable heat-insulating refractory liners provided hitherto invariably necessarily lack some of the key fundamental qualities required to develop sufficient hot strength at preheatable or sub-casting temperatures for the typical range in which preheating times occur. Consequently, there are strong commercial needs for preheatable heat-insulating refractory liners for metal casting vessels that can initiate the development of hot strength at preheat temperatures, that can withstand preheat temperatures for extended periods of preheat time, that can withstand molten metal erosion and corrosion, that will not experience substantial shrinkage on use, and that has minimum amounts of free silica and hydrogen content.